1. EE 122: Encoding And Framing Ion Stoica September 9, 2002

2. istoica@cs.berkeley.edu2 Overview  Encoding  Framing

3. istoica@cs.berkeley.edu3 Encoding  Goal: send bits from one node to another node on the same physical media -This service is provided by the physical layer  Problem: specify a robust and efficient encoding scheme to achieve this goal Adaptor Signal Adaptor: convert bits into physical signal and physical signal back into bits

4. istoica@cs.berkeley.edu4 Assumptions  We use two discrete signals, high and low, to encode 0 and 1  The transmission is synchronous, i.e., there is a clock used to sample the signal -In general, the duration of one bit is equal to one or two clock ticks

5. istoica@cs.berkeley.edu5 Non-Return to Zero (NRZ)  1  high signal; 0  low signal 0 01010110 NRZ (non-return to zero) Clock  Disadvantages: when there is a long sequence of 1’s or 0’s -Sensitive to clock skew, i.e., difficult to do clock recovery -Difficult to interpret 0’s and 1’s (baseline wander)

6. istoica@cs.berkeley.edu6 Non-Return to Zero Inverted (NRZI)  1  make transition; 0  stay at the same level  Solve previous problems for long sequences of 1’s, but not for 0’s 0 01010110 Clock NRZI (non-return to zero intverted)

7. istoica@cs.berkeley.edu7 Manchester  1  high-to-low transition; 0  low-to-high transition  Addresses clock recovery and baseline wander problems  Disadvantage: needs a clock that is twice as fast as the transmission rate 0 01010110 Clock Manchester

8. istoica@cs.berkeley.edu8 4-bit/5-bit  Goal: address inefficiency of Manchester encoding, while avoiding long periods of low or high signals  Solution: -Use 5 bits to encode every sequence of four bits such that no 5 bit code has more than one leading 0 and two trailing 0’s -Use NRZI to encode the 5 bit codes 0000 11110 0001 01001 0010 10100 0011 10101 0100 01010 0101 01011 0110 01110 1111 01111 1000 10010 1001 10011 1010 10110 1011 10111 1100 11010 1101 11011 1110 11100 1111 11101 4-bit 5-bit

9. istoica@cs.berkeley.edu9 Overview  Encoding  Framing

10. istoica@cs.berkeley.edu10 Framing  Goal: send a block of bits (frames) between nodes connected on the same physical media -This service is provided by the data link layer  Use a special byte (bit sequence) to mark the beginning (and the end) of the frame  Problem: what happens if this sequence appears in the data payload?

11. istoica@cs.berkeley.edu11 Byte-Oriented Protocols: Sentinel Approach  STX – start of text  ETX – end of text  Problem: what if ETX appears in the data portion of the frame?  Solution -If ETX appears in the data, introduce a special character DLE (Data Link Escape) before it -If DLE appears in the text, introduce another DLE character before it  Protocol examples -BISYNC, PPP, DDCMP Text (Data)STXETX 8 8

12. istoica@cs.berkeley.edu12 Byte-Oriented Protocols: Byte Counting Approach  Sender: insert the length of the data (in bytes) at the beginning of the frame, i.e., in the frame header  Receiver: extract this length and decrement it every time a byte is read. When this counter becomes zero, we are done

13. istoica@cs.berkeley.edu13 Bit-Oriented Protocols  Both start and end sequence can be the same -E.g., 01111110 in HDLC (High-level Data Link Protocol)  Sender: inserts a 0 after five consecutive 1s  Receiver: when it sees five 1s makes decision on the next two bits -if next bit 0 (this is a stuffed bit), remove it -if next bit 1, look at the next bit If 0 this is end-of-frame (receiver has seen 01111110) If 1 this is an error, discard the frame (receiver has seen 01111111) Text (Data) Start sequence 8 8 End sequence

14. istoica@cs.berkeley.edu14 Clock-Based Framing (SONET)  SONET (Synchronous Optical NETwork)  Example: SONET ST-1: 51.84 Mbps

15. istoica@cs.berkeley.edu15 Clock-Based Framing (SONET)  First two bytes of each frame contain a special bit pattern that allows to determine where the frame starts  No bit-stuffing is used  Receiver looks for the special bit pattern every 810 bytes -Size of frame = 9x90 = 810 bytes 90 columns 9 rows Data (payload) overhead SONET STS-1 Frame

16. istoica@cs.berkeley.edu16 Clock-Based Framing (SONET)  Details: -Overhead bytes are encoded using NRZ -To avoid long sequences of 0’s or 1’s the payload is XOR-ed with a special 127-bit patter with many transitions from 1 to 0

17. istoica@cs.berkeley.edu17 Summary  Encoding – specify how bits are transmitted on the physical media  Challenge – achieve: -Efficiency – ideally, bit rate = clock rate -Robust – avoid de-synchronization between sender and receiver when there is a large sequence of 1’s or 0’s  Framing – specify how blocks of data are transmitted  Challenge -Decide when a frame starts/ends -Differentiate between the true frame delimiters and delimiters appearing in the payload data